1. Field of the Invention
The present invention relates to 6-O-substituted benzoxazole and benzothiazole CSF-1R inhibitory compounds, their oxides, esters, prodrugs, solvates, or pharmaceutically acceptable salts thereof. This invention also relates to compositions of the compounds together with pharmaceutically acceptable carriers. In another aspect, this invention relates to uses of the compounds, either alone or in combination with at least one additional therapeutic agent, in the prophylaxis or treatment of cancer.
2. State of the Art
CSF-1R is the receptor for M-CSF (macrophage colony stimulating factor, also called CSF-1) and mediates the biological effects of this cytokine (Sherr 1985). The cloning of the colony stimulating factor-1 receptor (also called c-fms) was described for the first time in Roussel et al., Nature 325:549-552 (1987). In that publication, it was shown that CSF-1R had transforming potential dependent on changes in the C-terminal tail of the protein including the loss of the inhibitory tyrosine 969 phosphorylation which binds Cbl and thereby regulates receptor down regulation (Lee 1999).
CSF-1R is a single chain, transmembrane receptor tyrosine kinase (RTK) and a member of the family of immunoglobulin (Ig) motif containing RTKs characterized by repeated Ig domains in the extracellular portion of the receptor. The intracellular protein tyrosine kinase domain is interrupted by a unique insert domain that is also present in the other related RTK class III family members that include the platelet derived growth factor receptors (PDGFR), stem cell growth factor receptor (c-Kit) and fms-like cytokine receptor (FLT3). In spite of the structural homology among this family of growth factor receptors, they have distinct tissue-specific functions. CSF-1R is mainly expressed on cells of the monocytic lineage and in the female reproductive tract and placenta. In addition expression of CSF-1R has been reported in Langerhans cells in skin, a subset of smooth muscle cells (Inaba 1992), B cells (Baker 1993) and microglia (Sawada 1990).
The main biological effects of CSF-1R signaling are the differentiation, proliferation, migration, and survival of the precursor macrophages and osteoclasts from the monocytic lineage. Activation of CSF-1R is mediated by its only ligand, M-CSF. Binding of M-CSF to CSF-1R induces the formation of homodimers and activation of the kinase by tyrosine phosphorylation (Stanley 1997). Further signaling is mediated by the p85 subunit of PI3K and Grb2 connecting to the PI3K/AKT and Ras/MAPK pathways, respectively. These two important signaling pathways can regulate proliferation, survival and apoptosis. Other signaling molecules that bind the phosphorylated intracellular domain of CSF-1R include STAT1, STAT3, PLCγ, and Cbl (Bourette 2000).
CSF-1R signaling has a physiological role in immune responses, in bone remodeling and in the reproductive system. The knockout animals for either M-CSF-1 (op/op mouse; Pollard 1996) or CSF-1R (Dai 2002) have been shown to have osteopetrotic, hematopoietic, tissue macrophage, and reproductive phenotypes consistent with a role for CSF-1R in the respective cell types.
The recent success of Herceptin® and Avastin® has underscored the importance in developing therapeutics targeting a specific biological target. These drugs can minimize adverse events, have greater predictability, give physicians greater flexibility in their treatments, and provide researchers with a better understanding of a particular target. Additionally, targeted therapy may allow treatment of multiple indications affected by the same signaling pathway with fewer and potentially easier to manage toxicities. (BioCentury, V. 14(10) February, 2006) Inhibition of an individual kinase, such as CSF-1R, which is integrated within a pathway associated with cancer or other diseases, can effectively modulate downstream kinases as well, thereby affecting the entire pathway. However, the active sites of 491 human protein kinase domains are highly conserved, which makes the design of selective inhibitors a formidable challenge (Cohen 2005). Accordingly, there is a need for selective kinase inhibitors, such as selective CSF-1R inhibitors.